[REF] Reorganize submodules

[tsalo]

Closes None. References #220.

Changes proposed in this pull request:

• Move annotate submodules up a level.
• Consolidate Correctors and base class in one file.
• Remove model-based CBMA file. The current skeleton does not reflect what will ultimately be added to NiMARE.
• Remove Decoder base class. Our decoders are currently all functions.
• Change KernelTransformer argument "masked" to "return_type" to match MetaResult
• Fix thresholding bug in ALESubtraction and simplify interface
  • No meaningful threshold was being applied in the ALESubtraction. This change applies the threshold with a new parameter and allows users to simply provide the map name instead of the actual maps.

[codecov]

Codecov Report

Merging #225 into master will decrease coverage by 0.41%.
The diff coverage is 93.08%.

@@            Coverage Diff             @@
##           master     #225      +/-   ##
==========================================
- Coverage   61.06%   60.64%   -0.42%     
==========================================
  Files          71       65       -6     
  Lines        4418     4498      +80     
==========================================
+ Hits         2698     2728      +30     
- Misses       1720     1770      +50     

Impacted Files	Coverage Δ
nimare/annotate/utils.py	100.00% <ø> (ø)
nimare/decode/continuous.py	38.59% <ø> (ø)
nimare/decode/discrete.py	22.96% <ø> (ø)
nimare/decode/encode.py	82.75% <ø> (ø)
nimare/meta/cbma/__init__.py	100.00% <ø> (ø)
nimare/meta/cbma/peaks2maps.py	1.75% <ø> (ø)
nimare/meta/cbma/kernel.py	76.54% <70.37%> (-1.03%)	⬇️
nimare/meta/cbma/mkda.py	90.52% <88.88%> (ø)
nimare/meta/cbma/ale.py	93.35% <92.85%> (+0.30%)	⬆️
nimare/annotate/__init__.py	100.00% <100.00%> (ø)
... and 13 more

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